ASUS motherboards - "Last boot unsuccessful"

I've got a couple of AMD systems on ASUS motherboards, and they've
recently started complaining that the "last boot was unsuccessful" when
rebooting via 'shutdown -r'. This only started happening recently (in
the last month or so). Since I can no longer reliably reboot these
machines, it slows down testing immensely - I can no longer test while
I'm remote (ie, at the day job!).

Does anyone know what the ASUS BIOS might be checking to make this
determination? And is it something we an control?